Mechanism to preclude load replays dependent on off-die control element access in an out-of-order processor

ABSTRACT

An apparatus including first and second reservation stations. The first reservation station dispatches a load micro instruction, and indicates on a hold bus if the load micro instruction is a specified load micro instruction directed to retrieve an operand from a prescribed resource other than on-core cache memory. The second reservation station is coupled to the hold bus, and dispatches one or more younger micro instructions therein that depend on the load micro instruction for execution after a number of clock cycles following dispatch of the first load micro instruction, and if it is indicated on the hold bus that the load micro instruction is the specified load micro instruction, the second reservation station is configured to stall dispatch of the one or more younger micro instructions until the load micro instruction has retrieved the operand. The plurality of non-core resources includes a control element, coupled to the out-of order processor via a control bus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following co-pending patentapplications, each of which has a common assignee and common inventors.

SERIAL FILING NUMBER DATE TITLE MECHANISM TO PRECLUDE LOAD REPLAYS(VAS.2727-PCT) DEPENDENT ON FUSE ARRAY ACCESS IN AN OUT- OF-ORDERPROCESSOR MECHANISM TO PRECLUDE I/O-DEPENDENT LOAD (VAS.2728-PCT)REPLAYS IN AN OUT-OF-ORDER PROCESSOR MECHANISM TO PRECLUDE UNCACHEABLE-(VAS.2729-PCT) DEPENDENT LOAD REPLAYS IN OUT-OF-ORDER PROCESSOR LOADREPLAY PRECLUDING MECHANISM (VAS.2789-PCT) MECHANISM TO PRECLUDE LOADREPLAYS (VAS.3000-PCT) DEPENDENT ON PAGE WALKS IN AN OUT-OF- ORDERPROCESSOR MECHANISM TO PRECLUDE LOAD REPLAYS (VAS.3001-PCT) DEPENDENT ONLONG LOAD CYCLES IN AN OUT- OF-ORDER PROCESSOR PROGRAMMABLE LOAD REPLAYPRECLUDING (VAS.3002-PCT) MECHANISM POWER SAVING MECHANISM TO REDUCE(VAS.3005-PCT) LOAD REPLAYS IN OUT-OF-ORDER PROCESSOR MECHANISM TOPRECLUDE SHARED (VAS.3007-PCT) RAM-DEPENDENT LOAD REPLAYS IN AN OUT-OF-ORDER PROCESSOR APPARATUS AND METHOD TO PRECLUDE NON- (VAS.3008-PCT)CORE CACHE-DEPENDENT LOAD REPLAYS IN AN OUT-OF-ORDER PROCESSOR APPARATUSAND METHOD TO PRECLUDE X86 (VAS.3010-PCT) SPECIAL BUS CYCLE LOAD REPLAYSIN AN OUT-OF- ORDER PROCESSOR APPARATUS AND METHOD TO PRECLUDE LOAD(VAS.3011-PCT) REPLAYS DEPENDENT ON WRITE COMBINING MEMORY SPACE ACCESSIN AN OUT-OF-ORDER PROCESSOR APPARATUS AND METHOD FOR PROGRAMMABLE(VAS.3012-PCT) LOAD REPLAY PRECLUSION

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates in general to the field of microelectronics, andmore particularly to a power saving mechanism to reduce load replays inan out-of-order processor.

Description of the Related Art

Integrated device technologies have exponentially advanced over the past40 years. More specifically directed to the microprocessor fields,starting with 4-bit, single instruction, 10-micrometer devices, theadvances in semiconductor fabrication technologies have enableddesigners to provide increasingly more complex devices in terms ofarchitecture and density. In the 80's and 90's so-called pipelinemicroprocessors and superscalar microprocessors were developedcomprising millions of transistors on a single die. And now 20 yearslater, 64-bit, 32-nanometer devices are being produced that havebillions of transistors on a single die and which comprise multiplemicroprocessor cores for the processing of data.

In addition to the employment of instruction parallelism in present daymulti-core processors, out-of-order execution mechanisms are alsoprevalent. According to out-of-order execution principles, instructionsare queued in reservation stations for execution by execution units, andonly those instructions that are waiting on an operand as a result ofthe execution of older instructions are held up in the reservationstations; instructions that are not waiting on operands are dispatchedfor execution. Following execution, results are queued and put back intoregisters in proper order, typically in a processor stage called aretire state. Hence, the instructions are executed out of the originalprogram order.

Out-of-order execution provides for significant throughput improvementsince execution units, which would otherwise be sitting idle, areemployed to execute younger instructions while older instructions awaittheir operands. Though, as one skilled in the art will appreciate,instructions don't always execute successfully and, as a result, when agiven instruction does not execute successfully, that instruction andall instructions that are younger than that instruction must be executedagain. This concept is known as “replay,” because mechanisms in presentday processors essentially stop current execution, back up the machinestate to the point just prior to when the instruction executedunsuccessfully, and replay the unsuccessfully executed instruction alongwith all younger instructions, which may or may not have been dispatchedprior to dispatch of the unsuccessfully executed instruction.

Replay, however, is an exceptional case, and the performance impacts ofreplays is very often negligible. Yet, the performance impact of holdinginstructions in reservation stations until their operands are availableis significant, and microprocessor designers have developed accelerationtechniques that allow certain instructions to be dispatched when thereis a high probability that their operands will become available justprior to execution. Not only are these certain instructions dispatched,but mechanisms are put in place to provide their required operands justin time.

This application addresses one such acceleration technique where youngerinstructions that require an operand that is assumed with a highprobability to be resident in an on-core cache memory are dispatchedfollowing a specified number of clock cycles after dispatch of a loadinstruction whose execution leads to retrieval of the operand from thecache. Accordingly, when the load instruction is dispatched, the youngerinstructions that are waiting on its operand are stalled in theirrespective reservation stations until the specified number of clockcycles have transpired, and then the younger instructions are dispatchedfor execution with high certainty that their required operand willbecome available.

The performance improvement resulting from utilization of the abovenoted acceleration technique is so substantial that microprocessorarchitects typically apply the techniques across the board to all loadinstructions (e.g., loads from I/O, uncacheable loads, loads frominterrupt registers, special loads, etc.), even though it is certainthat there are a number of load instructions that will take longer thanthe specified number of cycles to obtain their operand, thus requiring areplay of all younger instructions that were dispatched in anticipationthat the operand would be available. The performance improvementsresulting from this load acceleration technique more than offset theperformance penalties incurred by infrequent replays.

But as multi-core processor technologies continue to advance, designersare now finding that certain processor resources, such as level 2 (L2)caches, interrupt controllers, fuse arrays, etc., which are infrequentlyaccessed, are better suited for placement in a common area of amulti-core processor die rather than being replicated within each of thecores. Hence, resources such as those noted above, are shared by theprocessor cores. As one skilled in the art will appreciate, to load anoperand from an off-core resource (say, a fuse array) takessubstantially longer than is required to load from an on-core resource(say, an L1 cache). And even though the performance penalty that isincurred as a consequence of having to perform replays of youngerinstructions that were dispatched under the above acceleration techniqueis not substantial, it has been observed by the present inventors thatthe power utilization impact is notable, for a remarkable number ofinstructions are being executed under conditions where it virtuallycertain that they will be replayed. And the initial execution of theseinstructions essentially wastes power, thus being disadvantageous fromthe standpoints of battery life, thermal profile, and reliability.

Therefore, what is needed is an apparatus and method that enables powerto be saved in a processor by reducing the number of replays that arerequired.

In addition, what is needed is a load replay reduction mechanism in anout-of-order processor that results in power savings for the processor.

SUMMARY OF THE INVENTION

The present invention, among other applications, is directed to solvingthe above-noted problems and addresses other problems, disadvantages,and limitations of the prior art.

One aspect of the present invention contemplates an apparatus forreducing replays in an out-of-order processor. The apparatus includes afirst reservation station and a second reservation station. The firstreservation station is configured to dispatch a first load microinstruction, and is configured to detect and indicate on a hold bus ifthe first load micro instruction is a specified load micro instructiondirected to retrieve an operand from a prescribed resource other thanon-core cache memory. The second reservation station is coupled to thehold bus, and is configured to dispatch one or more younger microinstructions therein that depend on the first load micro instruction forexecution after a first number of clock cycles following dispatch of thefirst load micro instruction, and if it is indicated on the hold busthat the first load micro instruction is the specified load microinstruction, the second reservation station is configured to stalldispatch of the one or more younger micro instructions until the firstload micro instruction has retrieved the operand. The plurality ofnon-core resources includes a control element, coupled to the out-oforder processor via a control bus.

Another aspect of the present invention comprehends an apparatus forreducing replays. The apparatus includes a multi-core processor, thathas a plurality of cores. Each of the plurality of cores includes afirst reservation station and a second reservation station. The firstreservation station is configured to dispatch a first load microinstruction, and is configured to detect and indicate on a hold bus ifthe first load micro instruction is a specified load micro instructiondirected to retrieve an operand from a prescribed resource other thanon-core cache memory. The second reservation station is coupled to thehold bus, and is configured to dispatch one or more younger microinstructions therein that depend on the first load micro instruction forexecution after a first number of clock cycles following dispatch of thefirst load micro instruction, and if it is indicated on the hold busthat the first load micro instruction is the specified load microinstruction, the second reservation station is configured to stalldispatch of the one or more younger micro instructions until the firstload micro instruction has retrieved the operand. The plurality ofnon-core resources includes a control element, coupled to the multi-coreprocessor via a control bus.

A further aspect of the present invention considers a method forreducing replays in an out-of-order processor. The method includes:disposing a plurality of non-core resources, comprising a controlelement, coupled to the out-of order processor via a control bus; via afirst reservation station, dispatching a first load micro instruction,and detecting and indicating on a hold bus if the first load microinstruction is a specified load micro instruction directed to retrievean operand from a prescribed resource other than on-core cache memory;and via a second reservation station that is coupled to the hold bus,dispatching one or more younger micro instructions therein that dependon the first load micro instruction for execution after a first numberof clock cycles following dispatch of the first load micro instruction,and if it is indicated on the hold bus that the first load microinstruction is the specified load micro instruction, stalling dispatchof the one or more younger micro instructions until the first load microinstruction has retrieved the operand.

Regarding industrial applicability, the present invention is implementedwithin a MICROPROCESSOR which may be used in a general purpose orspecial purpose computing device.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features, and advantages of the presentinvention will become better understood with regard to the followingdescription, and accompanying drawings where:

FIG. 1 is a block diagram illustrating a present day multi-coreprocessor that utilizes common resources disposed external to each ofthe cores;

FIG. 2 is a block diagram depicting exemplary core stages in each of thepresent day cores of FIG. 1;

FIG. 3 is a block diagram featuring a multi-core processor according tothe present invention that has a power saving mechanism for loads fromnon-core resources;

FIG. 4 is a block diagram showing exemplary cores stages in each of thecores of FIG. 3;

FIG. 5 is a block diagram illustrating details of the uncore stallelement of FIG. 4;

FIG. 6 is a block diagram detailing details of each of the reservationstations of FIG. 4; and

FIG. 7 is a block diagram showing details of the uncore miss element ofFIG. 4.

DETAILED DESCRIPTION

Exemplary and illustrative embodiments of the invention are describedbelow. In the interest of clarity, not all features of an actualimplementation are described in this specification, for those skilled inthe art will appreciate that in the development of any such actualembodiment, numerous implementation specific decisions are made toachieve specific goals, such as compliance with system related andbusiness related constraints, which vary from one implementation toanother. Furthermore, it will be appreciated that such a developmenteffort might be complex and time-consuming, but would nevertheless be aroutine undertaking for those of ordinary skill in the art having thebenefit of this disclosure. Various modifications to the preferredembodiment will be apparent to those skilled in the art, and the generalprinciples defined herein may be applied to other embodiments.Therefore, the present invention is not intended to be limited to theparticular embodiments shown and described herein, but is to be accordedthe widest scope consistent with the principles and novel featuresherein disclosed.

The present invention will now be described with reference to theattached figures. Various structures, systems, and devices areschematically depicted in the drawings for purposes of explanation onlyand so as to not obscure the present invention with details that arewell known to those skilled in the art. Nevertheless, the attacheddrawings are included to describe and explain illustrative examples ofthe present invention. The words and phrases used herein should beunderstood and interpreted to have a meaning consistent with theunderstanding of those words and phrases by those skilled in therelevant art. No special definition of a term or phrase (i.e., adefinition that is different from the ordinary and customary meaning asunderstood by those skilled in the art) is intended to be implied byconsistent usage of the term or phrase herein. To the extent that a termor phrase is intended to have a special meaning (i.e., a meaning otherthan that understood by skilled artisans) such a special definition willbe expressly set forth in the specification in a definitional mannerthat directly and unequivocally provides the special definition for theterm or phrase.

DEFINITIONS

Integrated Circuit (IC): A set of electronic circuits fabricated on asmall piece of semiconductor material, typically silicon. An IC is alsoreferred to as a chip, a microchip, or a die.

Central Processing Unit (CPU): The electronic circuits (i.e.,“hardware”) that execute the instructions of a computer program (alsoknown as a “computer application” or “application”) by performingoperations on data that include arithmetic operations, logicaloperations, and input/output operations.

Microprocessor: An electronic device that functions as a CPU on a singleintegrated circuit. A microprocessor receives digital data as input,processes the data according to instructions fetched from a memory(either on-die or off-die), and generates results of operationsprescribed by the instructions as output. A general purposemicroprocessor may be employed in a desktop, mobile, or tablet computer,and is employed for uses such as computation, text editing, multimediadisplay, and Internet browsing. A microprocessor may also be disposed inan embedded system to control a wide variety of devices includingappliances, mobile telephones, smart phones, and industrial controldevices.

Multi-Core Processor: Also known as a multi-core microprocessor, amulti-core processor is a microprocessor having multiple CPUs (“cores”)fabricated on a single integrated circuit.

Instruction Set Architecture (ISA) or Instruction Set: A part of acomputer architecture related to programming that includes data types,instructions, registers, addressing modes, memory architecture,interrupt and exception handling, and input/output. An ISA includes aspecification of the set of opcodes (i.e., machine languageinstructions), and the native commands implemented by a particular CPU.

x86-Compatible Microprocessor: A microprocessor capable of executingcomputer applications that are programmed according to the x86 ISA.

Microcode: A term employed to refer to a plurality of microinstructions. A micro instruction (also referred to as a “nativeinstruction”) is an instruction at the level that a microprocessorsub-unit executes. Exemplary sub-units include integer units, floatingpoint units, multimedia (MMX) units, and load/store units. For example,micro instructions are directly executed by a reduced instruction setcomputer (RISC) microprocessor. For a complex instruction set computer(CISC) microprocessor such as an x86-compatible microprocessor, x86instructions are translated into associated micro instructions, and theassociated micro instructions are directly executed by a sub-unit orsub-units within the CISC microprocessor.

Fuse: A conductive structure typically arranged as a filament which canbe broken at select locations by applying a voltage across the filamentand/or current through the filament. Fuses may be deposited at specifiedareas across a die topography using well known fabrication techniques toproduce filaments at all potential programmable areas. A fuse structureis blown (or unblown) subsequent to fabrication to provide for desiredprogrammability of a corresponding device disposed on the die.

In view of the above background discussion on load mechanisms inmulti-core processors and associated techniques employed within presentday multi-core processors to perform load operations from non-coreresources, a discussion of the limitations and disadvantages of thosepresent day techniques will be presented with reference to FIGS. 1-2.Following this, a discussion of the present invention will be presentedwith reference to FIGS. 3-7.

Turning to FIG. 1, a block diagram 100 is presented illustrating apresent day multi-core processor that utilizes common resources disposedexternal to each of the cores 101. The diagram 100 depicts a device die110, upon which is disposed four processor cores 101. The presentinventors note that for purposes of clarity, a quad-core (i.e., fourcores 101) multi-core microprocessor will be described herein, however,the principles and features of the present invention may be applied tomulti-core microprocessors having different numbers of cores 101.

As one skilled in the art will appreciate, for design and/or businessrelated reasons, architects may choose to share certain processorresources among the cores 101. For performance motives, these sharedresources are typically disposed on the same die 110 as the cores 101,and they are accessed by each of the cores 101 via high speed busses111-114. Accordingly, the diagram 100 shows exemplary shared resourcessuch as an L2 cache 103, random access memory (RAM) 104, a bus unit 105,an input/output unit 106, an advanced programmable interrupt controller(APIC) 107, and a fuse array 108. In aggregation, these shared resources103-108 that are disposed on the same die 110 as the cores 101, yetwhich are external to the cores 101, will be henceforth referred to asuncore resources 102. Thus, bus B1 111 enables CORE1 101 access to theuncore 102. Bus B2 112 enables CORE2 101 access to the uncore 102. BusB3 113 enables CORE3 101 access to the uncore 102. And bus B4 114enables CORE4 101 access to the uncore 102. In a typical configuration,the multi-core processor is coupled to other elements external to thedevice die 110 such as system memory (accessed via a memory bus MEM),input/output elements (accessed via bus I/O), and system controlelements (accessed via bus CTRL).

In operation, each of the cores 101, under operating system control, mayexecute associated instructions that are fetched from system memory, andwill perform operations on operands that correspond to intendedapplication. One or more of the cores 101 may require access to one ormore of the uncore resources 102 and will access the one or more of theuncore resources 102 in a controlled manner via a corresponding busB1-B4. For example, during power initialization, one or more of thecores 101 may perform a load operation from the fuse array 108 toretrieve configuration parameters, or may perform a load from the RAM104 to retrieve patch information. During normal operation, the cores101 may access the L2 cache 103 to read/write memory operands that arenot present in on-core caches (e.g., L1 caches). The cores 101 mayaccess the bus unit 105 to read/write from/to system memory, or mayutilize the I/O unit 106 to perform I/O operations via the I/O bus. Thecores 101 may furthermore access the APIC 107 to perform interruptoperations.

Now referring to FIG. 2, a block diagram is presented depictingexemplary core stages in each of the present day cores 101 of FIG. 1.The diagram shows a processor core 201 that is disposed on the die 110.The core 201 includes a fetch stage 211 that is coupled to a translatorstage 212 via bus 241. The translator stage 212 is coupled to a renamestage 213 via bus 242. The rename stage 213 is coupled to a replay muxstage 214 via bus 243. The replay mux 214 is coupled to a plurality ofreservation stations RS1-RSN 221.1-221.N and a load reservation stationRSL 221.L via reservation bus 244. Each of the reservation stationsRS1-RSN, RSL is coupled to a corresponding execution unit EU1-EUN222.1-222.N, EUL 222.L via a corresponding dispatch bus 252.1-251.N,251.L. The reservation stations RS1-RSN, RSL are coupled to a registerfile 226 via register bus 245.

The present inventors note that, outside of execution unit EUL, theremaining execution units EU1-EUN may comprise units typical to presentday superscalar processors such as integer units, floating point units,multimedia units, and store units. Of particular interest to the presentapplication is execution unit EUL, which is depicted as a load unit222.L whose primary function is to load operands from a variety ofresources such as system memory, system I/O, and uncore resources 230,as are described above with reference to FIG. 1.

Accordingly, the load unit EUL is coupled to an L1 cache 223 via bus 254and to the uncore 230 via bus 256. For most memory operands, the loadunit 222.L first accesses an L1 cache 223. If the load misses in the L1cache 223, then the load unit 222.L must access the L2 cache in theuncore 230. The execution units EU1-EUN, EUL are also coupled to areorder buffer 224 via bus 252. Additionally, the load unit EUL iscoupled to the reorder buffer 224 via bus MISS 253. The reorder buffer224 is coupled to the replay mux 214 via bus REPLAY 258 and to a retireunit 225 via bus 257. The retire unit 225 is coupled to the registerfile 226 via a writeback WB bus 255.

It is noted that the core stages shown in FIG. 2 are merely exemplary ofa present day superscalar, or “out-of-order” processor core 201, and arepresented for purposes of clearly teaching the present invention. As oneskilled in the art will appreciate, processor core stages may varyaccording to architecture and intended application.

In operation, program instructions (not shown) are fetched from memoryby the fetch unit 211. In an x86-compatible processor core 201, theseprogram instructions comport with the x86 ISA. The program instructionsare provided, in order, to the translator 212 on bus 241. The translator212 translates the program instructions into one or more microinstructions that direct sub-operations in corresponding execution unitsEU1-EUN, EUL in order to perform the operations specified by the programinstructions. The micro instructions are then provided to the renameunit 213 on bus 242, where architectural registers (i.e., operandregister locations) specified in some of the micro instructions areremapped to hardware registers (not shown) in the processor core 201 inorder to increase execution parallelism for independent microinstruction streams. The rename unit 213 also tags each of the microinstructions according to serial program order, and source anddestination operand fields in the micro instructions are also taggedwith the tags of the younger micro instructions upon which one or moreof their operands depend. The renamed micro instructions are thenprovided to the replay mux 214 on bus 243.

The replay mux 214 performs several functions in an out-of-orderprocessor core 201. Primarily, the mux 214 reads the opcodes in each ofthe renamed micro instructions to determine an appropriate executionunit EU1-EUN, EUL for execution. For example, renamed integer microinstructions may be executed by, say, EU1. Floating point microinstructions may be executed by EU2. And etc. And of particular interestto the present application, renamed load micro instructions may beexecuted by the load execution unit EUL. Thus, the replay mux 214provides one or more renamed micro instructions to one or more of thereservation stations RS1-RSN1, RSL to await dispatch to a correspondingexecution unit EU1-EUN, EUL.

Each of the reservation stations RS1-RSN, RSL accesses the register file226 to read operands required by their renamed micro instructions queuedfor operation therein. Renamed micro instructions that are not taggedwith the tags of older renamed micro instructions (i.e., the renamedmicro instructions are not dependent on the older renamed microinstructions) are immediately dispatched to corresponding executionunits EU1-EUN, EUL for execution. Dependent renamed micro instructions(i.e., renamed micro instructions that include tags of older renamedmicro instructions which have not yet completed execution) are generallyheld up by the reservation stations RS1-RSN, RSL until such time astheir tagged dependent operands are available. When the tagged dependentoperands are available, they are provided to the dependent renamed microinstructions, and the micro instructions are dispatched to correspondingexecution units EU1-EUN, EUL for execution. The execution units EU1-EUN,EUL may also perform a power saving function when they are not executingmicro instructions. Generally, clocks internal to the execution unitsEU1-EUN, EUL are shut down when they are not executing microinstructions, which saves substantial power.

Renamed micro instructions and their results are provided to the reorderbuffer 224 via bus 252. The reorder buffer places all results fromout-of-order execution of the renamed micro instructions back intoprogram order. That is, results from renamed programed registers areremapped back to their corresponding architectural registers and theresults are queued for entry in the architectural registers according tothe specified program order of execution. Micro instructions that havesuccessfully completed execution with proper results are provided to theretire unit 225 on bus 257. The results of these retired microinstructions are written back to the register file 226 on the WB bus255.

As one skilled in the art will appreciate, there are a number ofconditions that may cause a renamed micro instruction to executeunsuccessfully such as, but not limited to, program exceptions, generalinterrupts, I/O interrupts, branch exceptions, and the like. When thereorder buffer determines that a renamed micro instruction has notexecuted successfully, then it must be re-executed (“replayed”) alongwith all younger renamed micro instructions that have been dispatchedfor execution. Accordingly, the reorder buffer 224 initiates a replayevent by providing the tag of the unsuccessfully executed renamed microinstruction on the replay bus 258.

When an unsuccessfully executed renamed micro instruction tag isprovided to the replay mux 214, the replay mux 214 in response backs upmachine state to comport with execution of renamed micro instructionsbeginning at the renamed micro instruction whose tag was provided onREPLAY 258.

One skilled in the art will also appreciate that in order to improveperformance, microprocessor designers often make assumptions regardinghow instructions will execute. For example, it is well known in the artthat a significant percentage of branches will not be taken. Thus, thefetch unit 211 may be configured to queue up instructions for executionaccording to that assumption. If a branch is not taken, then overallexecution speed is improved. If the branch is taken, then allinstructions older than the branch must be replaced with instructionsfrom the taken program path.

Another assumption that microprocessor designers make is that a loadmicro instruction will hit in the L1 cache 223 in a specified number ofclock cycles, and this assumption is based upon L1 cache hit statistics,say 90 percent hit rate, along with the number of clock cycles requiredaccording to design to access the L1 cache 223. For purposes of thepresent application, consider that it will take four clock cycles toaccess the L1 cache 223, however, such a number is chosen forinstructional purposes. Other numbers of clock cycles are contemplated.

Accordingly, a reservation station RS1-RSN may comprise logic thatstalls renamed micro instructions whose tags correspond with an olderload micro instruction until four clock cycles after dispatch of theolder load instruction, and then dispatches the renamed microinstructions to their corresponding execution units EU1-EUN under theassumption that the older load micro instruction will hit in the L1cache 223 within the four clock cycles and the tagged operand will beready. Though not shown in FIG. 2, it is noted that the execution unitsEU1-EUN, EUL may also access operands made available from loadoperations and provide these operands to micro instructions currentlyunder execution. For loads that hit in the L1 cache 223, the operandsare provided to the dispatched dependent younger micro instructions,which execute through completion much faster than would otherwise beprovided for. But for loads that miss in the L1 cache (approximately 10percent of the time according to a 90 percent hit rate assumption),after successful completion of the loads, all dependent younger microinstructions that were dispatched under the assumption of a hit must bereplayed. Thus, upon a miss in the L1 cache 232, the load execution unitEUL will inform the reorder buffer 224 of such by indicating the tag ofthe missed load instruction on bus MISS 253 to initiate the replay ofthe younger dependent instructions.

Such a scheme is incredibly effective from a performance standpoint, formost present day caches 223 are very efficient, and thus it is a commonpractice to stall all micro instructions that depend from a load microinstruction for a number of clock cycles after dispatch of the loadinstruction based on a presumed cache access number of clock cycles(e.g., four clock cycles). The dependent micro instructions are stalledin their respective reservation station RS1-RSN, and then dispatchedwhen it is assumed that the operands specified in the load microinstruction are available from the L1 cache 223. Typically, thisacceleration scheme is utilized for all load instructions, includingload instructions that access resources other than the L1 cache 223.Because these types of load instructions are relatively infrequentcompared to memory load instructions, the performance impact associatedwith replay of load instructions that access resources other than memoryis generally tolerated. As such, when a load micro instruction takeslonger than a specified number of clock cycles (four clock cycles inthis example) to successfully execute (i.e., to “resolve”), then theload execution unit EUL will declare a miss on bus MISS, thus causingdependent younger micro instructions to be replayed after completion ofthe load.

The above technique has served well to improve the performance ofsuperscalar processors 201 over the past several years, but the presentinventors have observed that additional challenges are presented whenthis scheme is adapted to a multi-core processor configuration, such asis shown in FIG. 1. More specifically, such as scheme, while extremelyeffective in configurations dominated by access to an L1 cache 223, isnot very power efficient when applied to a multi-core processorconfiguration that increasingly utilizes accesses to uncore resources230, for access times to uncore resources 230 are very slow whencompared to those of a present day L1 cache 223.

What this means is that any load micro instruction that is specificallydirected to an uncore resource 230 such as the fuse array 108, the busunit 105 (for uncacheable loads), the APIC 107, the I/O unit 106, andperhaps the L2 cache 103 and RAM 104, will result in replays ofdependent younger micro instructions having the tags of those loads fromthe uncore resources 230. And the present inventors have observed that,while not much of a performance hit, the wasted initial executions ofthese dependent younger micro instructions result in power expendituresthat are significant, because load micro instructions—which areguaranteed to be replayed—are being dispatched to execution unitsEU1-EUN, thus utilizing power that would otherwise be saved by powermanagement mechanisms therein.

The present invention overcomes the above noted limitations of presentday load mechanisms, and others, by providing an apparatus and methodfor saving power in an out-of-order multi-core processor by reducing thenumber of load replays according to a novel scheme, which will now bediscussed with reference to FIGS. 3-7.

Referring now to FIG. 3, a block diagram 300 is presented featuring amulti-core processor according to the present invention that has a powersaving mechanism for loads from non-core resources. The diagram 300depicts a device die 310, upon which is disposed four processor cores301. It is noted that four cores 301 are shown only for purposes ofclearly teaching the present invention, and the principles and detailsdiscussed below can be applied to processors having any number of cores301 where access times for certain load operations from non-coreresources are longer than those for on-core cache access.

Like the multi-core processor of FIG. 1, the multi-core processoraccording to the present invention may uncore resources 302 that aretypically disposed on the same die 310 as the cores 301, and they areaccessed by each of the cores 301 via high speed busses 311-314.Accordingly, the diagram 300 shows exemplary shared resources such as,but not limited to, an L2 cache 303, random access memory (RAM) 304, abus unit 305, an input/output unit 306, an APIC 307, and a fuse array308. Thus, bus B1 311 enables CORE1 301 access to the uncore 302. Bus B2312 enables CORE2 301 access to the uncore 302. Bus B3 313 enables CORE3301 access to the uncore 302. And bus B4 314 enables CORE4 301 access tothe uncore 302. In a typical configuration, the multi-core processor iscoupled to other elements (not shown) external to the device die 310such as, but not limited to, system memory (accessed via a memory busMEM), input/output elements (accessed via bus I/O), and system controlelements (accessed via bus CTRL). The control elements may include, butare not limited to, a Peripheral Component Interconnect Express (PCI-e)element, a Peripheral Component Interconnect (PCI) element, a UniversalSerial Bus (USB) element, a graphics adaptor, a co-processor, and aninter-processor communications element.

In contrast to the multi-core processor of FIG. 1, the multi-coreprocessor according to the present invention includes a replay reducerelement 320 within each of the cores 301. In one embodiment, the replayreducer 320 is configured to detect loads that are directed to resourcesother than on-core cache memory (not shown), to stall all youngerdependent micro instructions from being dispatched until the loadsresolve, and to preclude assertion of any indications that wouldotherwise result in a replay event. Thus, one or more execution units(not shown) within the cores 301 may enter into a power management modeas a result of stalling dispatch of the younger dependent microinstructions, thus saving power on the die 310 which would otherwise bewasted.

In operation, each of the cores 301, under operating system control, mayexecute associated instructions that are fetched from system memory, andwill perform operations on operands that correspond to intendedapplication. One or more of the cores 301 may require access to one ormore of the uncore resources 302 and will access the one or more of theuncore resources 302 in a controlled manner via a corresponding busB1-B4. For example, during power initialization, one or more of thecores 301 may perform a load operation from the fuse array 308 toretrieve configuration parameters, or may perform a load from the RAM304 to retrieve microcode patches and/or other configurationinformation. During normal operation, the cores 301 may access the L2cache 303 to read/write memory operands which may have been cached fromthe system memory that are not present in on-core caches (e.g., L1caches). The cores 301 may access the bus unit 305 to read/write from/tosystem memory, or may utilize the I/O unit 306 to perform I/O operationsvia the I/O bus. The cores 302 may access the bus unit 305 to read/writecontrol data from/to the control elements. The cores 301 may furthermoreaccess the APIC 307 to perform interrupt operations.

Rather than automatically declaring a miss as a result of these loadsfrom uncore 302, resulting in replay of dependent younger microinstruction streams, the replay reducer 320 will stall execution of thedependent younger micro instruction streams until the loads resolve,thus enabling execution unit power management functions to be utilized.In one embodiment, the replay reducer 320 may also detect other types ofloads that are not specifically directed to uncore resources 302, butwhich are guaranteed to otherwise cause a miss indication. These othertypes of loads may include, but are not limited to, I/O loads, loadsrequiring a specified number of cycles, loads that are known to requirea page table walk such as those associated with second level addresstranslation (i.e., nested paging, x86 extended page table loads), loadsresulting from execution of x86 special bus cycles (e.g., shutdown,halt, flush, etc.), and loads that are known to resolve to uncacheablememory space or write combining space. Other embodiments contemplatedetection of any type of load operation where there is a significantprobability that it will take more than the specified number of clockcycles to complete.

Turning to FIG. 4, a block diagram 400 is presented depicting exemplarycore stages in each of the cores 301 of FIG. 3. The diagram shows aprocessor core 401 that is disposed on the die 310. The core 401includes a fetch stage 411 that is coupled to a translator stage 412 viabus 441. The translator stage 412 is coupled to a rename stage 413 viabus 442. The rename stage 413 is coupled to a replay mux stage 414 viabus 443. The replay mux 414 is coupled to a plurality of reservationstations RS1-RSN 421.1-421.N and an enhanced load reservation stationERSL 421.L via reservation and hold bus HOLDY 444. Reservation stationERSL includes an uncore stall element 461. Each of the reservationstations RS1-RSN, ERSL is coupled to a corresponding execution unitEU1-EUN 422.1-422.N, EUL 422.L via a corresponding dispatch bus451.1-451.N, 451.L. The reservation stations RS1-RSN, ERSL are coupledto a register file 426 via register bus 445.

Except for execution unit EUL, the remaining execution units EU1-EUN maycomprise units typical to present day superscalar processors such asinteger units, floating point units, multimedia units, and store units.Execution unit EUL is a load unit 422.L whose primary function is toload operands from a variety of resources including, but not limited to,system memory, system I/O, and uncore resources 430, as are describedabove with reference to FIG. 3. Execution unit EUL additionallycomprises an unmiss element UMISS 462.

Accordingly, the load unit EUL is coupled to an L1 cache 423 via bus 454and to the uncore 430 via bus 456. For memory operands, the load unit422.L first accesses an L1 cache 423. If the load misses in the L1 cache423, then the load unit 422.L must access the L2 cache (not shown) inthe uncore 430. The execution units EU1-EUN, EUL are also coupled to areorder buffer 424 via bus 452. Additionally, the load unit EUL iscoupled to the reorder buffer 424 via bus MISS 453. The reorder buffer424 is coupled to the replay mux 414 via bus REPLAY 458, to a retireunit 425 via bus 457, and to the reservation stations RS1-RSN, ERSL viathe HOLDY bus 444. The retire unit 425 is coupled to the register file426 via a writeback WB bus 455.

It is noted that the core stages shown in FIG. 4 are presented asexamples in order to clearly teach aspects of the present invention,because they are exemplary of those within present day out-of-orderprocessor cores, yet is it noted that one skilled in the art will beable to adapt aspects and features of the present invention describedherein to other processor core stage configurations which may berequired according to architecture and intended application.

In operation, program instructions (not shown) are fetched from memory(not shown) by the fetch unit 411. In an x86-compatible processor core401, these program instructions comport with the x86 ISA. The programinstructions are provided, in order, to the translator 412 on bus 441.The translator 412 translates the program instructions into one or moremicro instructions that direct sub-operations in corresponding executionunits EU1-EUN, EUL in order to perform the operations specified by theprogram instructions. The micro instructions are then provided to therename unit 413 on bus 442, where architectural registers (i.e., operandregister locations) specified in some of the micro instructions areremapped to hardware registers (not shown) in the processor core 401 inorder to increase execution parallelism for independent microinstruction streams. The rename unit 413 also tags each of the microinstructions according to serial program order, and source anddestination operand fields in the micro instructions are also taggedwith the tags of the younger micro instructions upon which one or moreof their operands depend. The renamed micro instructions are thenprovided to the replay mux 414 on bus 443.

The replay mux 414 reads the opcodes in each of the renamed microinstructions to determine an appropriate execution unit EU1-EUN, EUL forexecution and, in particular, renamed load micro instructions areexecuted by the load execution unit EUL. Thus, the replay mux 414 thusprovides one or more renamed micro instructions to one or more of thereservation stations RS1-RSN1, ERSL to await dispatch to a correspondingexecution unit EU1-EUN, EUL.

Each of the reservation stations RS1-RSN, ERSL accesses the registerfile 426 to read operands required by their renamed micro instructionsqueued for operation therein. Renamed micro instructions that are nottagged with the tags of older renamed micro instructions (i.e., therenamed micro instructions are not dependent on the older renamed microinstructions) are immediately dispatched to corresponding executionunits EU1-EUN, EUL for execution. Except as will be described below,dependent younger renamed micro instructions (i.e., renamed microinstructions that include tags of older renamed micro instructions whichhave not yet completed execution) are generally held up by thereservation stations RS1-RSN, ERSL until such time as their taggeddependent operands are available. When the tagged dependent operands areavailable, they are provided to the dependent younger renamed microinstructions, and the younger micro instructions are dispatched tocorresponding execution units EU1-EUN, EUL for execution. The executionunits EU1-EUN, EUL may also perform a power saving function when theyare not executing micro instructions. Clocks internal to the executionunits EU1-EUN, EUL are shut down when they are not executing microinstructions, which saves substantial power.

Renamed micro instructions and their results are provided to the reorderbuffer 424 via bus 452. The reorder buffer 424 places all results fromout-of-order execution of the renamed micro instructions back intoprogram order. That is, results from renamed programed registers areremapped back to their corresponding architectural registers and theresults are queued for entry in the architectural registers according tothe specified program order of execution. Micro instructions that havesuccessfully completed execution with proper results are provided to theretire unit 425 on bus 457. The results of these retired microinstructions are written back to the register file 426 on the WB bus455.

When the reorder buffer 424 determines that a renamed micro instructionhas not executed successfully, then it must be replayed along with allyounger dependent renamed micro instructions that have been dispatchedfor execution. Accordingly, the reorder buffer 424 initiates a replayevent by providing the tag of the unsuccessfully executed renamed microinstruction on the replay bus 458.

When an unsuccessfully executed renamed micro instruction tag isprovided to the replay mux 414, the replay mux 414 in response backs upmachine state to comport with execution of renamed micro instructionsbeginning at the renamed micro instruction whose tag was provided onREPLAY 458.

Except as will be described below, the present invention comprisesreservation stations RS1-RSN that are configured to stall microinstructions that depend on a younger load micro instruction therein fora specified number of clock cycles after the load micro instruction isdispatched, presuming the load micro instruction will hit in the L1cache 423 in the specified number of clock cycles. In one embodiment,the specified number of clock cycles is four clock cycles. Other numbersof clock cycles are contemplated.

Accordingly, except as is described below, the reservation stationsRS1-RSN comprise logic that stalls renamed micro instructions whose tagscorrespond with an older load micro instruction until four clock cyclesafter dispatch of the older load instruction, and then dispatches therenamed younger micro instructions to their corresponding executionunits EU1-EUN under the assumption that the load micro instruction willhit in the L1 cache 423 within the four clock cycles and the taggedoperand will be ready. Though not shown in FIG. 4, it is noted that theexecution units EU1-EUN, EUL may also access operands made availablefrom load operations and provide those operands to micro instructionsexecuting therein. For loads that complete in less than the specifiednumber of cycles, such as loads that hit in the L1 cache 423, theoperands are provided to the dispatched dependent younger microinstructions, which execute through completion much faster than wouldotherwise be provided for. And for loads take more than the specifiednumber of clock cycles, such as those that miss in the L1 cache, aftersuccessful completion of the loads, all dependent younger microinstructions that were dispatched under the assumption of a hit must bereplayed. Thus, upon a miss in the L1 cache 423, the load execution unitEUL will inform the reorder buffer 424 of such by indicating the tag ofthe missed load instruction on bus MISS 453 to initiate the replay ofthe younger instructions.

The present invention, however, also provides for exceptions to theabove acceleration scheme by including uncore stall logic 461 in theenhanced load reservation station ERSL 421.L which detects one or moreload micro instruction types in order to implement power savings in oneor more of the execution units EU1-EUN by stalling younger microinstructions that are dependent on micro instructions of the one or moreload micro instruction types until their operands are available. Theunmiss logic 462 also detects the one or more load micro instructiontypes in order to preclude assertion of a miss on bus MISS 453 when themicro instructions of the one or more load micro instruction types takemore than the specified number of clock cycles to obtain their operands.By doing so, the micro instructions of the one or more load microinstruction types are allowed to complete execution, and replays ofthose younger micro instructions that are dependent on the microinstructions of the one or more micro instruction types are not requiredbecause those younger dependent micro instructions have been stalled inthe reservation stations RS1-RSN. In one embodiment, the reservationstations RS1-RSN, ERSL communicate information (e.g., tags) associatedwith detected micro instructions of the one or more micro instructiontypes to each other and to the reorder buffer 424 via bus HOLDY 444 inorder to initiate stalls of the younger dependent micro instructions.When the micro instructions of the one or more load micro instructiontypes complete execution, the reorder buffer 424 directs the reservationstations RS1-RSN to release their stalled younger dependent microinstructions for dispatch by providing tags of the micro instructions ofthe one or more load micro instruction types that have completed onHOLDY 444.

Advantageously, the present invention provides for effective performancerelated to load micro instructions corresponding to operands cached fromsystem memory, and substantially reduces the number of replaysassociated with load micro instructions of the one or more microinstruction types, thus enabling the execution units EU1-EUN to enterinto power savings modes as they become empty due the implementeddependency stalls.

Thus, for instance, a load micro instruction that is specificallydirected to an uncore resource 430 such as the fuse array 308, the busunit 305, the APIC 307, the I/O unit 306, the L2 cache 303 and the RAM304, will not result in replays of younger dependent micro instructionshaving the tags of those loads from the uncore resources 430.

In one embodiment, the one or more load micro instruction types mayinclude loads from specified uncore resources 430, along with othertypes of loads including, but not limited to, I/O loads, loads requiringa specified number of cycles, loads from system memory that are known torequire a page table walk, loads resulting from execution of x86 specialbus cycles (e.g., shutdown, halt, flush, etc.), loads that are known toresolve to uncacheable memory space, and loads that are known to resolveto write combining memory space. Other embodiments contemplate detectionof any type of load operation where there is a significant probabilitythat it will take more than the specified number of clock cycles tocomplete.

In one embodiment, the uncore stall element 461 and the unmiss element462 may be configured upon initialization (e.g., power up or reset) ofthe processor core 401 according to the present invention for detectionof prescribed load micro instruction types. The prescribed load microinstruction types may be read upon initialization from specifiedlocations in the fuse array 308. In another embodiment, each of thecores 401 may be configured to detect different types of prescribed loadmicro instructions via programming in the fuse array 308, where typesassociated with each of the cores 401 may be programmed intocorresponding locations in the fuse array 308. In a further embodiment,the prescribed load micro instruction types may be programmed into theRAM 304 upon power up or reset via a Joint Test Action Group (JTAG)interface (not shown) to the multi-core device 310, where the prescribedload micro instruction types are read upon subsequent initializationfrom specified locations in the RAM 304.

Now referring to FIG. 5, a block diagram 500 is presented illustratingdetails of the uncore stall element 461 of FIG. 4. The stall element 461comprises a micro instruction register 510 that is coupled to uncoreload opcodes detect logic 501. The micro instruction register 510comprises an micro instruction tag field OP TAG 511, an opcodes fieldMICRO OP 512, a source A field SRC A 513, a tag A field TAG A 514, asource B field SRC B 515, a tag B field TAG B 516, a source C field SRCC 517, and a tag C field TAG C 518. The detect logic 501 generates ahold signal HOLDY that is coupled to bus 444.

As one skilled in the art will appreciate, present day ISAs, such as thex86 ISA, provide for a number of different operand addressing modesincluding, but not limited to, direct, indirect, immediate, andrelative. Consequently, one or more of the source fields SRC A-C maycontain operands, one or more may specify locations of operands(including destination of result). Consequently, operation of the stallelement 461 will be described in a generic sense as regards the contentsof the source fields SRC A-C to allow for a broader application of thepresent invention to a wide number of ISAs.

Operationally, as micro instructions are provided by the replay mux 414,load micro instructions are entered into the micro instruction register510. OP TAG has the tag of the current micro instruction in the register510, MICRO OP has its opcodes. Contents of TAG A may contain the tag ofan older micro instruction upon which contents of SRC A depend. Contentsof TAG B may contain the tag of an older micro instruction upon whichcontents of SRC B depend. Contents of TAG C may contain the tag of anolder micro instruction upon which contents of SRC C depend. The detectlogic 501 is configured to read contents of MICRO OP. If MICRO OP doesnot contain one of the prescribed load opcodes that are described abovewhich will cause younger micro instructions in other reservationstations RS1-RSN that depend upon the current micro instruction in theregister 510 to be stalled, then the detect logic 501 deasserts HOLD Y,thus indicating to RS1-RSN that those younger micro instructions thereinmay be dispatched in due course. If, however, MICRO OP does contain oneof the prescribed load opcodes that are described above which will causeyounger micro instructions in other reservation stations RS1-RSN thatdepend upon the current micro instruction in the register 510 to bestalled, then the detect logic 501 asserts HOLD Y and places contents ofOP TAG on bus 444, thus indicating to RS1-RSN that those youngerdependent micro instructions therein must be stalled until the loadprescribed by the currently micro instruction in register 510 iscompleted and a result of the load is provided to the younger dependentmicro instructions. When the load is completed, the reorder buffer 424will deassert HOLD Y, thus releasing the stall.

Turning to FIG. 6, a block diagram is presented showing details of eachof the reservation stations RS1-RSN of FIG. 4. The reservation stationcomprises a micro instruction register 610 that is coupled to dependencycheck logic 601. The micro instruction register 510 comprises an microinstruction tag field OP TAG 611, an opcodes field MICRO OP 612, asource A field SRC A 613, a tag A field TAG A 614, a source B field SRCB 615, a tag B field TAG B 616, a source C field SRC C 617, and a tag Cfield TAG C 618. The dependency check logic 601 generates a ready signalREADY and monitors hold signal HOLDY that is coupled to bus 444.

Contents of the fields 611-618 in register 610 are the same as isdescribed above with reference to FIG. 5 for like-named fields. Thecheck logic 601 is additionally configured to read contents of thesource tag fields TAG A-C. If contents of any of the tag fields TAG A-Cmatch a tag that is on HOLDY when asserted, then the micro instructionwithin the register 610 is stalled until such time as the load microinstruction upon which the micro instruction within the register 610depends completes, its operand obtained via the load is provided to acorresponding source field SRC A-C, and the reorder buffer 424 deassertsHOLDY. When HOLD Y is deasserted, the check logic 601 asserts READY,indicating that the micro instruction in register 610 is ready to bedispatched to its corresponding execution unit EU1-EUN.

If contents of the tag fields TAG A-C do not match a tag that is onHOLDY when asserted, then the check logic 601 asserts READY, indicatingthat the micro instruction in register 610 is ready to be dispatched toits corresponding execution unit EU1-EUN.

Referring now to FIG. 7, a block diagram 700 is presented showingdetails of the uncore miss element 462 of FIG. 4. The uncore misselement 462 comprises a micro instruction register 710 that is coupledto load miss preclusion logic 701. The micro instruction register 710comprises an micro instruction tag field OP TAG 711, an opcode fieldMICRO OP 712, a source A field SRC A 713, a tag A field TAG A 714, asource B field SRC B 715, a tag B field TAG B 716, a source C field SRCC 717, and a tag C field TAG C 718. The miss preclusion logic 501generates a no miss signal NOMISS.

Contents of the fields 711-718 in register 710 are the same as isdescribed above with reference to FIGS. 5-6 for like-named fields. Thepreclusion logic 701 is configured to read contents of MICRO OP. IfMICRO OP does not contain one of the prescribed load opcodes that aredescribed above which will cause younger dependent micro instruction tobe stalled, then the load miss preclusion logic 701 deasserts signalNOMISS, which informs corresponding load execution unit EUL 422.L tomanage the state of signal MISS in according to normal load instructionexecution protocols. If MICRO OP contains one of the prescribed opcodes,then the preclusion logic 701 asserts NOMISS, which informs thecorresponding load execution unit EUL 422.L to preclude assertion ofMISS during execution of the micro instruction in register 710.

The elements described above according to the present invention areconfigured to perform the functions and operations as discussed herein.The present invention elements comprises logic, circuits, devices, ormicrocode (i.e., micro instructions or native instructions), or acombination of logic, circuits, devices, or microcode, or equivalentelements that are employed to execute the functions and operationsaccording to the present invention as noted. The elements employed toaccomplish these operations and functions may be shared with othercircuits, microcode, etc., that are employed to perform other functionsand/or operations within the multi-core microprocessor.

Portions of the present invention and corresponding detailed descriptionare presented in terms of software, or algorithms and symbolicrepresentations of operations on data bits within a computer memory.These descriptions and representations are the ones by which those ofordinary skill in the art effectively convey the substance of their workto others of ordinary skill in the art. An algorithm, as the term isused here, and as it is used generally, is conceived to be aself-consistent sequence of steps leading to a desired result. The stepsare those requiring physical manipulations of physical quantities.Usually, though not necessarily, these quantities take the form ofoptical, electrical, or magnetic signals capable of being stored,transferred, combined, compared, and otherwise manipulated. It hasproven convenient at times, principally for reasons of common usage, torefer to these signals as bits, values, elements, symbols, characters,terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise, or as is apparent from the discussion,terms such as “processing” or “computing” or “calculating” or“determining” or “displaying” or the like, refer to the action andprocesses of a computer system, a microprocessor, a central processingunit, or similar electronic computing device, that manipulates andtransforms data represented as physical, electronic quantities withinthe computer system's registers and memories into other data similarlyrepresented as physical quantities within the computer system memoriesor registers or other such information storage, transmission or displaydevices.

Note also that the software implemented aspects of the invention aretypically encoded on some form of program storage medium or implementedover some type of transmission medium. The program storage medium may beelectronic (e.g., read only memory, flash read only memory, electricallyprogrammable read only memory), random access memory magnetic (e.g., afloppy disk or a hard drive) or optical (e.g., a compact disk read onlymemory, or “CD ROM”), and may be read only or random access. Similarly,the transmission medium may be metal traces, twisted wire pairs, coaxialcable, optical fiber, or some other suitable transmission medium knownto the art. The invention is not limited by these aspects of any givenimplementation.

The particular embodiments disclosed above are illustrative only, andthose skilled in the art will appreciate that they can readily use thedisclosed conception and specific embodiments as a basis for designingor modifying other structures for carrying out the same purposes of thepresent invention, and that various changes, substitutions andalterations can be made herein without departing from the scope of theinvention as set forth by the appended claims.

What is claimed is:
 1. An apparatus for reducing replays in anout-of-order processor, the apparatus comprising: a first reservationstation, configured to dispatch a first load micro instruction, andconfigured to detect and indicate on a hold bus if said first load microinstruction is one of a plurality of specified load instructionsdirected to one of a plurality of non-core resources; a secondreservation station, coupled to said hold bus, configured to dispatchone or more younger micro instructions therein that depend on said firstload micro instruction for execution after a first number of clockcycles following dispatch of said first load micro instruction, and ifit is indicated on said hold bus that said first load micro instructionis said one of a plurality of specified load instructions, said secondreservation station is configured to stall dispatch of said one or moreyounger micro instructions until said first load micro instruction hasretrieved said operand; and said plurality of non-core resources,comprising: a control element, coupled to the out-of order processor viaa control bus.
 2. The apparatus as recited in claim 1, wherein theout-of-order processor comprises a multi-core processor, and whereineach core within said multi-core processor comprises said first andsecond reservation stations.
 3. The apparatus as recited in claim 2,wherein said one of said plurality of non-core resources is disposed onthe same die as said each core, yet is disposed external to said eachcore.
 4. The apparatus as recited in claim 2, wherein said one of aplurality of non-core resources comprises said control element, andwherein said control element is not disposed on the same die as saidmulti-core processor, and wherein said control element is accessed via abus unit disposed on the same die as each core, yet disposed external tosaid each core.
 5. The apparatus as recited in claim 1, furthercomprising: load execution logic, coupled to said first reservationstation, configured to receive and execute said first load microinstruction, and configured to enter a power savings state if no microinstructions are received for execution.
 6. The apparatus as recited inclaim 5, wherein, if said first load micro instruction is not saidspecified load micro instruction, said load execution logic indicates ona miss bus if said first load micro instruction fails to successfullyexecute in said first number of clock cycles, thus initiating a replayof said one or more younger micro instructions.
 7. The apparatus asrecited in claim 6, wherein, if said first load micro instruction issaid specified load micro instruction, said load execution logic doesnot indicate that said first load micro instruction fails tosuccessfully execute if more than said first number of clock cycles arerequired to successfully execute, thus precluding a replay of said oneor more younger micro instructions.
 8. An apparatus for reducingreplays, the apparatus comprising: a multi-core processor, comprising aplurality of cores, wherein each of said plurality of cores comprises: afirst reservation station, configured to dispatch a first load microinstruction, and configured to detect and indicate on a hold bus if saidfirst load micro instruction is one of a plurality of specified loadinstructions directed to one of a plurality of non-core resources; asecond reservation station, coupled to said hold bus, configured todispatch one or more younger micro instructions therein that depend onsaid first load micro instruction for execution after a first number ofclock cycles following dispatch of said first load micro instruction,and if it is indicated on said hold bus that said first load microinstruction is said one of a plurality of specified load instructions,said second reservation station is configured to stall dispatch of saidone or more younger micro instructions until said first load microinstruction has retrieved said operand; and said plurality of non-coreresources, comprising: a control element, coupled to said multi-coreprocessor via a control bus.
 9. The apparatus as recited in claim 8,wherein said multi-core processor comprises an x86-compatible multi-coreprocessor.
 10. The apparatus as recited in claim 8, wherein said one ofsaid plurality of non-core resources is disposed on the same die as saidmulti-core processor, yet is disposed external to said each of saidplurality of cores.
 11. The apparatus as recited in claim 8, whereinsaid one of said plurality of non-core resources comprises said controlelement, and wherein said control element is not disposed on the samedie as said multi-core processor, and wherein said control element isaccessed via a bus unit disposed on the same die as each core, yetdisposed external to said each core.
 12. The apparatus as recited inclaim 8, wherein said each of said plurality of cores further comprises:load execution logic, coupled to said first reservation station,configured to receive and execute said first load micro instruction, andconfigured to enter a power savings state if no micro instructions arereceived for execution.
 13. The apparatus as recited in claim 12,wherein, if said first load micro instruction is not said specified loadmicro instruction, said load execution logic indicates on a miss busthat said first load micro instruction fails to successfully executewhen more than said first number of clock cycles are required tosuccessfully execute, thus initiating a replay of said one or moreyounger micro instructions.
 14. The apparatus as recited in claim 13,wherein, if said first load micro instruction is said specified loadmicro instruction, said load execution logic does not indicate that saidfirst load micro instruction fails to successfully execute when morethan said first number of clock cycles are required to successfullyexecute, thus precluding a replay of said one or more younger microinstructions.
 15. A method for reducing replays in an out-of-orderprocessor, the method comprising: disposing a plurality of non-coreresources, comprising a control element, that is coupled to the out-oforder processor via a control bus; via a first reservation station,dispatching a first load micro instruction, and detecting and indicatingon a hold bus if the first load micro instruction is a one of theplurality of specified load instructions directed one of the pluralityof non-core resources; and via a second reservation station that iscoupled to the hold bus, dispatching one or more younger microinstructions therein that depend on the first load micro instruction forexecution after a first number of clock cycles following dispatch of thefirst load micro instruction, and if it is indicated on the hold busthat the first load micro instruction is the one of the plurality ofspecified load instructions, stalling dispatch of the one or moreyounger micro instructions until the first load micro instruction hasretrieved the operand.
 16. The method as recited in claim 15, whereinthe out-of-order processor comprises a multi-core processor, and whereineach core within the multi-core processor comprises the first and secondreservation stations.
 17. The method as recited in claim 16, wherein theone of the plurality of non-core resources is disposed on the same dieas the each core, yet is disposed external to the each core.
 18. Themethod as recited in claim 16, wherein the one of the plurality ofnon-core resources comprises the control element, and wherein thecontrol element is not disposed on the same die as the out-of-orderprocessor, and wherein the control element is accessed via a bus unitdisposed on the same die as each core, yet disposed external to the eachcore.
 19. The method as recited in claim 15, further comprising: viaload execution logic that is coupled to the first reservation station,receiving and executing the first load micro instruction, and entering apower savings state if no micro instructions are received for execution.20. The method as recited in claim 19, wherein, if the first load microinstruction is not the specified load micro instruction, indicating on amiss bus that the first load micro instruction fails to successfullyexecute when more than the first number of clock cycles are required tosuccessfully execute, thus initiating a replay of the one or moreyounger micro instructions.
 21. The method as recited in claim 20,wherein, if the first load micro instruction is the specified load microinstruction, not indicating that the first load micro instruction failsto successfully execute when more than the first number of clock cyclesare required to successfully execute, thus precluding a replay of theone or more younger micro instructions.